1. Field of the Invention
This invention relates to an inkjet print head, and more particularly, to an inkjet print head having ejection ports for ejecting different ink drops.
2. Description of the Related Art
For halftone reproduction, some inkjet printing methods employ a dot density control method for controlling the number of print dots per unit area by the print dot of a uniform size. In a known printing method of them, ejection ports for ejecting ink drops of different sizes are provided in order to eject the small ink drops to form print dots for a part of an image ranging from a light tone to a half tone, and to eject the lager ink drops to form print dots for a part of the image ranging from a half tone to a dark tone (see Japanese Patent Laid-open No. H04-10941 (1992), for example).
In a known printing apparatus in which ejection ports are designed to eject ink drops of different sizes as described above, the ejection ports are arranged such that ink paths are changed in cross-sectional area and/or ink-flow resistance for large fluid drops and small fluid drops (see Japanese Patent Laid-open No. 2003-311964, for example).
On the other hand, if the size of the ink drop is more reduced for an improvement in image quality, a desired amount of ink ejection may not be applied because of the small ink drops. To avoid this, the resolution of a row of ejection ports can be increased with a reduction in size of the ink drop. In this case, however, the ratio of the size of a heater to the resolution of the row of the ejection ports significantly increases. This makes it difficult to route heater wiring, which in turn may make it impossible to arrange heaters in line. Also, the ink paths for supplying ink may not be arranged in line.
Therefore, the zigzag arrangement of the heaters as shown in FIG. 10 is generally known. Also, the print head with ejection ports for ejecting large and small ink drops which are arranged in a zigzag relationship is known (see Japanese Patent Laid-Open 2005-1379, for example).
For printing by the inkjet printing method, the ink in the ejection port is rapidly heated by the heater, to create a bubble. The expansion of the bubble forces the ink to drop out of the ejection port. In this printing, sub droplets (satellites) following the main drop at the time of drop formation may cause image degradation. Specifically, depending on the directionality of an ink tail formed at the time of drop formation, the flying direction of the satellites is changed. As a result, the satellites and the main drop fly in different directions from each other. For example, when the ink paths for ejecting small ink drops differ in length by arranging the ejection ports in a zigzag relationship, the flying pattern of the satellites may be varied in accordance with the ink-path length. For this reason, in the print head with the zigzag arrangement of the ejection ports, the landing of the satellites may affect a printed image. For example, it may cause an increase in graininess of the printed image and/or inconsistencies in density or a streak on a scan boundary because of a difference in dot density.
For the purpose of limiting the effect of the deviation of the landing position on the print image, the printing speed can be reduced by reducing the speed of the carriage moving in the main scan direction or by increasing the number of multi-paths, in order to lower the effect of the satellites. However, this method cannot offer an improvement in printing speed.
In addition, as the size of a droplet is increasingly reduced, the satellite droplets may disadvantageously cause occurrence of stains in the inside of the printing apparatus such as a printer, due to misting.